1. Field of the Invention
The present invention relates to a thin-film transistor circuit utilized as a driver circuit for a liquid crystal display apparatus or for an imaging sensor, and particularly, to a thin-film transistor circuit suitable for a driver-monolithic active matrix type liquid crystal display apparatus.
2. Description of the Related Art
A logic gate circuit, such as NOT, NOR, NAND, etc., consisting of thin-film transistors is conventionally used as a driver for a liquid crystal display apparatus. FIG. 5A is a schematic plan view illustrating a conventional thin-film transistor circuit 530. FIG. 5B is a schematic cross sectional view of the thin-film transistor circuit 530 taken along a line A-A' in FIG. 5A of the thin-film transistor circuit 530. The thin-film transistor circuit 530 is a NOT-type logic gate circuit.
The thin-film transistor circuit 530 has a load transistor 521 formed on an insulating substrate 501 and a driver transistor 522 formed on the insulating substrate 501. The load transistor 521 consists of a gate electrode 502 formed on the insulating substrate 501, a gate insulating film 504 formed on the insulating substrate 501 and the gate electrode 502, a channel region 505 formed on the gate insulating film 504, a source region 507 formed on the gate insulating film 504, and a drain region 508 formed on the gate insulating film 504. The channel region 505 is made of intrinsic amorphous silicon. The source region 507 and the drain region 508 are made of highly doped n-type (n.sup.+) amorphous silicon. The driver transistor 522 consists of a gate electrode 503 formed on the insulating substrate 501, the gate insulating film 504 formed on the insulating substrate 501 and the gate electrode 503, a channel region 506 formed on the gate insulating film 504, a source region 509 formed on the gate insulating film 504, and a drain region 510 formed on the gate insulating film 504. The channel region 506 is made of intrinsic amorphous silicon. The source region 509 and the drain region 510 are made of n.sup.+ amorphous silicon.
The gate electrode 502 of the load transistor 521 is electrically connected to a supply line 511 through a contact hole 512 formed in the gate insulating film 504. The source region 507 of the load transistor 521 is electrically connected to the supply line 511. Both the drain region 508 of the load transistor 521 and the source region 509 of the driver transistor 522 are electrically connected to an output line 513. The drain region 510 of the driver transistor 522 is electrically connected with a ground line 514.
Because the gate electrode 502 is electrically connected to the source region 507, the load transistor 521 is in an on state at any time and functions as a resistor. In the thin-film transistor circuit 530, when a low-level voltage is applied to the gate electrode 503 of the driver transistor 522, the driver transistor 522 turns off. A current, then, flows from the supply line 511 to the output line 513 via the load transistor 521. When a high-level voltage is applied to the gate electrode 503, the driver transistor 522 turns on. Then, a current flows from the supply line 511 to the ground line 514, and the output line 513 has the same voltage as the voltage of the ground line 514. Thus, the output line 513 can output a high-level voltage during the off state of the driver transistor 522 and a low-level voltage during the on state of the driver transistor 522 by adjusting the resistance of the load transistor 521.
In the aforementioned conventional thin-film transistor circuit 530, the load transistor 521 serves as a resistor device. In order to make the thin-film transistor circuit 530 work as a logic gate circuit, the current gain of the driver transistor 522 must be fifty times larger than that of the load transistor 521. Since thin-film transistors made of amorphous silicon generally have a small current gain, the driver transistor 522 must have a large channel region 506 so that the current gain of the driver transistor 522 is sufficiently large. Therefore, the driver transistor 522 must have a large area, which prevents the thin-film transistor circuit 530 from being made small. Other logic gate circuits than the NOT-type logic gate circuit may also have the same problems as mentioned above.
The present invention overcomes the aforementioned shortcomings associated with the conventional technique and provides a thin-film transistor circuit having a small area and good device characteristics.